1. Field of the Invention
The present invention relates to nickel base superalloys, especially wrought alloys of the gamma-gamma prime type having improved resistance to low cycle fatigue at intermediate elevated temperatures in gas turbine disc applications.
2. Description of the Prior Art
The low cycle fatigue capability of nickel base superalloy turbine disc materials has lately assumed a position of overriding importance in the design of gas turbine engines. Previously, a need for high performance engine materials having only a limited life, e.g., 8,500 cycles, placed an emphasis upon improving the tensile and creep properties of disc alloys. But now, with increasing emphasis on longer life 20,000 to 30,000 cycle design life engine components, it has been realized that dynamic (vibratory) properties such as fatigue life have not kept pace with the improvements in static properties such as creep strength, stress-rupture strength and tensile strength. As a result of this imbalance, gas turbine disc stress levels must be kept to a relatively low level to achieve long life, compared to the stress limitation imposed by the static properties alone.
Considerable study of low cycle fatigue in turbine discs has given insight into the stress-strain environment and material behavior. The principal fatigue load is imposed by stresses and temperature gradients which are produced during normal starting and stopping of the engine. For example, a gas turbine low fatigue cycle in an aircraft is defined as one takeoff and landing. (Low cycle fatigue is distinguished from high cycle fatigue which results from substantially lower stresses, such as occur periodically during rotation of the engine; these infrequently are design-limiting.) In general, disc components are limited in notched low cycle fatigue; certain regions affected by stress raisers, such as a hole, become locally plastic during operation, while the remainder of the disc remains elastic. Damage gradually accumulates in the plastic zone about the stress raiser leading to initiation of a microcrack. After initiation, crack growth occurs by crystallographic shear as cycles are further accumulated until the crack assumes a detactable size. The smallest crack that can be detected in situ by standard quality control methods is about 0.030 inch; a detectable crack of any size is usually cause for replacement of the disc.
Heretofore there has been considerable art evidencing improvements in nickel base alloys. Probably the major thrust has been on increasing temperature capability, by developing gamma-gamma prime high strength alloys. Generally, the objective has been to increasingly add alloying elements to increase and make uniform the gamma prime phase while simultaneously strengthening or hardening the gamma matrix phase. In most past instances, alloys which have been used for compressor or turbine discs are modifications of gamma prime strengthened nickel base alloys which were initially developed for use in turbine blades. For example, Waspaloy, Astroloy and IN-100 alloys, currently utilized as turbine disc materials, are all blade alloys slightly modified in chemistry to allow casting in ingot sizes sufficient to produce the much larger discs under forging. The modification of blade alloys has, until recently, proved a most successful method of approaching disc material development, since new and improved turbine blade materials generally have had higher temperature and strength capabilities, properties which have been taken advantage of in the disc application.
In recent years, however, the properties required of discs and blades have diverged significantly. As turbine inlet temperatures increased beyond the capability of superalloys, turbine blades have required air cooling. Since cooling affects the efficiency of the engine, the major emphasis in developing blade alloys has been on higher temperature capability. The cooling air for blades passes through the discs in which the blades are held and this has resulted in turbine disc temperatures seldom exceeding 1200.degree.-1300.degree. F. despite the higher temperatures the blades endure. Therefore, the high temperature (1600.degree.-1800.degree. F.) capability of blade alloys is unnecessary in the turbine discs operating at intermediate temperatures, which range from about 800.degree. F. to about 1300.degree. F., and typically are about 1200.degree. F. in the more demanding applications. However, at the same time, fatigue-causing thermal gradients and resultant thermal strains are produced in the discs in addition to cyclic mechanical stresses. Low cycle fatigue crack initiation and growth have consequently become very significant for discs. The dynamic properties of the blade-derived disc alloys have remained essentially constant despite improvements in strength, creep, and stress rupture capabilities. There is of course variation in fatigue resistance between different alloys, depending on their composition and processing method. However, the difference in fatigue resistance solely due to composition is not great compared to the three-fold or greater advance which is needed. Attempts have been made to improve the low cycle fatigue properties of alloys through thermomechanical processing--controlled sequencing of forging and heat treatment--which will, for example, vary the grain size. While finer grain size can improve fatigue life, a corollary can be an unacceptable reduction in creep strength. In addition, powder metallurgical techniques have been used either to avoid segregation during ingot formation in complex alloys having improved properties or to minimize grain size. Processing techniques necessary to achieve fine grain size or form powder metal objects require additional steps, equipment, and controls, which can be costly.